3.x: Cleanup and prettify Javadocs, widen XOperator throws

[akarnokd]

This PR clears up some javadocs and widens the throws on the various Operator interfaces:

• Link to Integer.MAX_VALUE and Long.MAX_VALUEs.
• Add links to types and {@code TheType} for other appearances.
• Wrap event type indicators into {@code } tags.
• Wrap boolean outcomes into {@code } tags.
• Wrap null mentions into {@code } tags.
• Apply nullability annotations.
• Fix wording and copy-paste errors.

[codecov]

Codecov Report

Merging #6785 into 3.x will decrease coverage by 0.01%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##                3.x    #6785      +/-   ##
============================================
- Coverage     98.24%   98.22%   -0.02%     
+ Complexity     6255     6253       -2     
============================================
  Files           690      690              
  Lines         45218    45218              
  Branches       6220     6220              
============================================
- Hits          44426    44417       -9     
- Misses          276      281       +5     
- Partials        516      520       +4

Impacted Files	Coverage Δ	Complexity Δ
...a/io/reactivex/rxjava3/observers/SafeObserver.java	100% <ø> (ø)	18 <0> (ø)	⬇️
...o/reactivex/rxjava3/processors/AsyncProcessor.java	98.95% <ø> (ø)	45 <0> (ø)	⬇️
...in/java/io/reactivex/rxjava3/schedulers/Timed.java	100% <ø> (ø)	13 <0> (ø)	⬇️
...eactivex/rxjava3/processors/BehaviorProcessor.java	96.58% <ø> (-2.44%)	51 <0> (ø)
...reactivex/rxjava3/processors/UnicastProcessor.java	100% <ø> (ø)	68 <0> (ø)	⬇️
.../main/java/io/reactivex/rxjava3/core/Flowable.java	100% <ø> (ø)	577 <0> (ø)	⬇️
...tivex/rxjava3/disposables/CompositeDisposable.java	98.14% <ø> (ø)	39 <0> (ø)	⬇️
...o/reactivex/rxjava3/core/BackpressureStrategy.java	100% <ø> (ø)	1 <0> (ø)	⬇️
...vex/rxjava3/exceptions/UndeliverableException.java	100% <ø> (ø)	1 <0> (ø)	⬇️
...ctivex/rxjava3/internal/util/QueueDrainHelper.java	98.61% <ø> (-1.39%)	57 <0> (-1)
... and 46 more

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